34 
BULLETIN OF THE LABORATORIES 
are narrowly flask-shaped or fusiform. The difference is emphasized 
in the sections by the fact that the gyrus uncinatus lies in a different 
plane from the gyrus fornicatus, and thus when the cells of the latter 
are cut longitudinally those of the former are almost always cut trans- 
versely. Perpendicular longitudinal sections of the mouse brain, how- 
ever, show the two laminae in much closer contact, and in the opos- 
sum the continuity is unbroken, especially cephalad, though in the 
caudal portion the arrangement is more like that of rodents. Old em- 
bryos of the rabbit, too, show the formation of the folds above 
described with great clearness. The gyrus uncinatus in these rodents 
thus appears to be more or less displaced, as well as folded. In the 
rabbit embryos referred to it would appear to originate as a thickening 
on the free edge of the gyrus fornicatus, which is thrust back as by 
some external resistance, both on the ectal and ental surfaces of this 
edge, but chiefly on the latter. The cells which connect the two por- 
tions at this stage appear to be subsequently lost or to be replaced by 
fibrous elements. 
The confluence of the gray matter of the two hippocampi, which 
Stieda describes in the case of the mouse, does not occur here. The 
external fibre zones of the two sides do come into contact in the 
meson, but this is obviously merely a mechanical adhesion. I suspect 
that the confluence which Stieda observed was more apparent than 
real, though my own mouse sections show the two sides in much 
closer contact than in Geomys. Fiber zibethicus resembles the mouse 
in this respect, and in the configuration of the hippocampus as a 
whole. 
Erethizon dorsatus presents a structure essentially like that de- 
scribed for Geomys, though somewhat more difficult to follow. The 
chief difference lies in the gyrus uncinatus, which is larger and over- 
laps the free margin of the gyrus fornicatus much more than in 
Geomys. In Arctomys monax there is considerable variation, though 
only in the relative position of the parts. It is illustrated quite fully 
in Vol. V, Plate IV, Fig. t, Plate V, Figs, i, 2, and 3, Plate VII, 
Fig. 2, Plate X, Figs. 5 and 7, Plate XIX, Figs, i and 2. 
Diencephalon and Mesencephalon. The relations here are essen- 
tially as in the rat. The thalamus is much smaller than in Erethizon, 
the cephalo-dorsal portion especially being abbreviated. Thecinereum 
is nearly circular with the infundibulum passing through the centre. 
The chiasm is not so prominent as in Erethizon, though essentially 
